A computer connected to a network is vulnerable to attack from other computers on that network. If the network is the Internet, the attacks may include a range of acts from malicious attempts to gain access to the computer, to installing “zombie” code, to denial of service attacks. Malicious attempts to gain access to the computer may have the intent of discovering personal data, while zombie code may be used to launch denial of service attacks by overwhelming a web site with high traffic volumes from a number of computers. The attackers may include organized criminals, sophisticated but malicious computer experts, and “script kiddies” who read and repeat posted assaults on known vulnerabilities.
Most computers have addressable ports for sending and receiving data. Some of the ports may be designated for certain kinds of traffic. For example, in an Internet Protocol (IP) network, port 80 is often designated for hyptertext protocol (http) traffic, while port 443 is often designated for secure http (https) traffic. Other ports may be designated as needed for different services. Non-designated traffic on such designated ports and any traffic on unused ports may indicate attempts by attackers to gain access to the computer.
A firewall may be used to limit port traffic to certain protocols and to close unused ports from all outside traffic. The firewall may be placed on a network between computers seeking protection and “open” networks, such as the Internet, or may be integral to the computer. In corporations, or other large private networks, firewalls may also be used to limit traffic between business units. The firewall may block traffic at a designated port having the wrong protocol, for example, file transfer protocol (FTP) may be blocked on port 80. Similarly, the firewall may block all traffic on an unused port. Both hardware and software implementations of firewalls are available.